In recent years, in order to reduce the workload of drivers of large trucks or buses, an automatic transmission apparatus which can automatically select a speed ratio corresponding to the driving conditions of a vehicle, has been proposed.
Conventional automatic transmission apparatuses have been developed for small vehicles, and normally comprise a gear position switch means for a planetary gear transmission, in which a fluid joint (e.g., a torque converter) is inserted between the engine and the planetary gear transmission, and compressed oil is used as the control medium.
In the development of automatic transmission apparatuses for large trucks, it is preferable to utilize a conventional drive system (e.g., a friction clutch, a transmission, and the like), as well as conventional product lines, since the number of such trucks manufactured is much smaller than that of small vehicles, and designing a new and expensive purpose-made torque converter increases cost.